The present invention relates to the field of fluid sampling and concerns a sampling device or system particularly adapted for use in a gas analysis process.
In all fields using a gas medium such as air separation processes, petroleum refining, natural gas production, semiconductor devices manufacturing, specialty gas laboratories, etc. all gas being processed or used in one way or another must be analyzed for quality control or process control. To perform such an analysis, a gas sample is collected and brought to an analytical measuring system. Generally, the gas sample is conveyed through metal tubing, up to a sample panel. A plurality of samples may need to be successively collected, depending on the complexity of a particular system. The analyzed sample should of course be representative of the gas medium being controlled.
The industry has used and still uses various devices and processes to bring a sample to an analytical system. With these sampling systems, contamination of a sample often occurs by mixing it with previously selected samples, leaks in or out of the sampling system or leaky valves.
FIG. 1 (prior art) illustrates a sampling process frequently used in older systems and is still in use today. In this system 10, various sample lines 12 made of various tubing material each bring a corresponding sample to the apparatus sample inlet 14. A plurality of sampling locations may be provided, as required by the process to be monitored. A bypass rotometer 16 is provided in each line, for purging a given sampling line 12 when not selected. The rotometer 16 allows fixing of a bypass flow and preferably sets a high flow in the sample line 12 to speed up the purge time. The excess flow is vented out of the system. A female quick connector 18 is provided at the extremity of each sampling line 12 and is adapted to receive a male quick connector 20 allowing the sample to flow through the flexible line 22 up to the analytical system 24. To change the selected sample line 12, the male quick connector 20 needs to be removed from the female quick connector 18 and inserted in another one. This system makes sure that there is no sample cross contamination from various sample points, since the sampling lines 12 are physically isolated. However, this system has serious drawbacks. First, each time the male quick connector 20 is disconnected from a female quick connector 18, the gas flow to the analytical system 24 is momentarily interrupted. Some analytical systems 24 are affected by the sample flow variation. Also the female and male quick connectors 18 and 20 have some internal dead volume that will be filled with atmospheric air when disconnected from each other. This air is directed to the analytical system and serious pollution may occur when measuring H2O, O2 or N2 as impurities in a particular background. Another drawback is that the quick connectors 18 and 20 tend to wear out with use, resulting in leaks leading to wrong analytical results. Another problem with this system is related to the use of flexible tubing. Often this tube is made of various plastic or polymers that exhibit too much permeation to O2 and H2O polluting the sample. When flexible metal tubing is used, it must be replaced often since metal fatigue due to manipulation causes them to break.
FIG. 2 (prior art) shows another sample stream selection used in the industry. This system 10 is similar to the one of FIG. 1, but uses instead of quick connectors, a rotary selection valve 26 well known in the industry and available from various manufacturers. This system 10 alleviates some of the drawbacks of the previous one, but introduces cross port flow contamination that increases with time. If a sample line 12 has a higher pressure, it will leak through the valve body 26 and pollute the stream being measured. This valve requires frequent replacement. Furthermore, leaks can occur from the valve stem.
FIG. 3 (prior art) shows another system used in the industry. In this system 10 each sampling line 12 includes an ON/OFF valve 28 provided downstream the bypass rotometer 16. However, this system introduces dead volume in the line section 30 downstream the valve 28. When switching from one sample to a new one, the line section of the previously selected sample is full of the previous sample. This gas is trapped there and will slowly diffuse in the line, slowing down the response time of the system and causing drifting readings of the analytical system 24. Another source of unswept dead volume is the valve itself. The space surrounding the valves plunger is always filled with sample gas and slowly diffuses in the main stream, causing measurement drift and noise. A Diaphragm based valve may be used to reduce the problem, but it increases the cost of the system since most of the time the use of such a valve will involve orbital welding for assembly. Furthermore, over time, ON/OFF valves will develop leaks. So an unselected stream may leak to a selected one, resulting in analytical error measurement and apparent drift or noise when the sample line pressure varies again. As soon as a valve develops a leak it must be replaced, interrupting the system in service. There are some variations of the previously described systems but all have similar drawbacks.
Also known in the art is U.S. Pat. No. 5,922,286 (GIRARD et al). Girard discloses a system that selects individual sample streams with the help of a 4-way, pneumatic actuated, VCR xc2xcxe2x80x3 connected diaphragm valve. Even if this system succeeds in eliminating unswept volume present on the discharge side of the valve and provides some means to have a sample inlet bypass flow or purge, it fails to eliminate the problem associated with leaking valves, i.e. crossport flow contamination. The selected sample must flow through all unselected valve bodies just around the seat, which is quite large. Therefore, the risk of crossport contamination increases with the number of sampling lines in the system. Diaphragms having a relatively short useful life there will eventually be leaking across the seat and contamination of the selected sample. Finally the diaphragm valves used in this system are costly and the total space required for this system is quite large.
Other related prior art systems include U.S. Pat. Nos. 5,054,309; 5,055,260; 5,065,794; 5,239,856; 5,259,233; 5,447,053; 5,587,519 and 5,661,225.
It is an object of the present invention to provide a fluid sampling system that prevents cross-port flow contamination between various sampling lines.
It is another object of the invention to provide such a system that may be manufactured inexpensively.
It is a preferable object of the present invention to provide such a system that is insensitive to valve leaks.
It is also a preferable object of the invention to provide such a system allowing an evacuation of dead volume from the valves used.
The present invention therefore provides a fluid sampling system for providing a fluid sample to a fluid processing apparatus. The sampling system has a sample outlet connected to said apparatus, and a plurality of sampling channels. Each of the sampling channels includes the following elements:
an inlet line receiving a fluid and an outlet line conveying the fluid to the sample outlet;
a valve connected to the inlet line and to the outlet line, the valve having a closed position preventing a fluid flow between the inlet line and the outlet line and an open position allowing a fluid flow between the inlet line and the outlet line;
a first purge line connected to the inlet line and purging fluid therefrom, and a second purge line connected to the outlet line and purging fluid therefrom; and
flow controlling means for controlling a fluid flow in the first and second purge lines.
The fluid sampling system further includes a connecting line for connecting the outlet lines of the sampling channels to each other and to the sample outlet, and valve control means for selectively setting the valve of one of the sampling channels in the opened position and the valves of the remaining sampling channels in the closed position.
Other features and advantages of the present invention will be better understood upon reading the following description of preferred embodiments thereof with reference to the appended drawings.